Isoprenoid compounds are organic molecules produced by a wide range of organisms (e.g., plants, bacteria, fungi, etc). To date, over 23,000 individual isoprenoid molecules have been characterized with tens to hundreds of new structures identified each year. These molecules can fulfill a variety of roles. For example, monoterpenes can be used as fragrances and flavors. Sesquiterpenes and diterpenes can serve as pheromones, defensive agents, visual pigments, antitumor drugs, and components of signal transduction pathways. Triterpenes can serve important functions as membrane constituents and precursors of steroid hormones and bile acids. Polyprenols function as photoreceptive agents and cofactor side chains, and can also exist as natural polymers.
The diverse molecular compounds produced by the isoprenoid pathway are created from diphosphate esters of monounsaturated isoprene units. Isoprenes are added together in multiples of 2, 3, or 4 by prenyl transferases to make C10, C15, and C20 units, respectively. The C10, C15, and C20 molecules, named geranyl diphosphate (GPP), farnesyl diphosphate (FPP), and geranylgeranyl diphosphate (GGPP), respectively, serve as substrates for terpene synthases.
Terpene synthases catalyze the production of isoprenoid compounds via one of the most complex reactions known in chemistry or biology. In general, terpene synthases are moderately sized enzymes having molecular weights of about 40 to 100 kD. As an enzyme, terpene synthases can be classified as having low to moderate turnover rates coupled with exquisite reaction specificity and preservation of chirality. Turnover comprises binding of substrate to the enzyme, establishment of substrate conformation, conversion of substrate to product and product release. Reactions can be performed in vitro in aqueous solvents, typically require magnesium ions as cofactors, and the resulting products, which are often highly hydrophobic, can be recovered by partitioning into an organic solvent.
Terpene synthase genes are found in a variety of organisms including bacteria, fungi and plants. Swapping regions approximating exons between different terpene synthases has identified functional domains responsible for terminal enzymatic steps. For example, work performed on 5-epi-adstolochene synthase (TEAS) from Nicotiana tabacum (tobacco) and Hyoscyamus muticus vetispiradiene synthase (HVS) from henbane revealed that exon 4 and exon 6, respectively, were responsible for reaction product specificity. Combining functional domains resulted in novel enzymes capable of synthesizing new reaction products (U.S. Pat. No. 5,824,774).
Studies have led to proposed reaction mechanisms for isoprenoid production; see, e.g., Cane et al., 1985, Bioorg. Chem., 13:246-265; Wheeler and Croteau, 1987, Proc. Natl. Acad. Sci. USA, 84:4856-4859; and Pyun et al., 1994, Arch. Biochem. Biophys., 308:488-496. The studies used substrate analogs and suicide inhibitors (Croteau, 1994, Arch. Biochem. Biophys., 251:777-782; Cane et al., 1995, Biochemistry, 34:2471-2479; and Croteau et al., 1993, Arch. Biochem. Biophys., 307:397-404), as well as chemical-modifying reagents and site-directed mutagenesis in efforts to identify amino acids essential for catalysis (Cane et al., 1995, Biochemistry, 34:2480-2488; Rajaonarivony et al., 1992, Arch. Biochem. Biophys., 296:49-57; and Rajaonarivony et al., 1992, Arch. Biochem. Biophys., 299:77-82). However, these studies have resulted in limited success in defining the active site due to inherent limitations with these techniques.